Piddocks – are anything but boring

Walking along the beach in Southern California, you might come across holey rocks and wonder, who or what in the world made all those holes? Perhaps the result of some alien testing to determine what planet Earth is all about? Or someone with way too much time on their hands? Not exactly…

The answer is: piddock clams!

Piddock clams are a type of clam that can bore into rocks. This happens to be an excellent strategy if you need to protect yourself from pounding waves. Instead of living with the constant threat of being washed away, it’s much safer and secure to create a permanent home in a rock.

Like many other marine animals, piddock clams start their lives as plankton, floating along at the mercy of ocean currents. While they are floating, piddock clams start looking for a place to settle by tasting and smelling the water to find fellow piddock clams that have already found a great rock home.

Once a piddock clam finds a suitable rock community of like-minded clams, it grasps the rock’s surface with a broad foot and starts to grow a shell. The shell has ridges or tooth-like serrations the piddock clam uses to burrow into the rock by twisting back and forth.

Piddock clams continue boring into the rock until they are fully grown. Then they secrete shell material over their foot and officially take up residence. But how do you eat if you can’t leave your rock home?

Luckily, piddock clams have two siphons. To eat, piddock clams pump in water and filter food from the water with one siphon. Then they pump out waste with the other siphon. This all takes place as they are snugly situated in a rock.

And there you have it! The reason you may find holey rocks.

Piddocks are bivalves, and their specially-adapted oval shells are edged with fine teeth which they use to excavate burrows in rock. Their fleshy foot grips the stone surface and helps to rotate the shell, creating a circular scouring action. Piddocks will also drill into submergedwood, if it happens to be available.

Once the piddock has carved a safe tunnel for itself, it settles down inside and extends a siphon through the entrance, which it uses to filter food such as phytoplankton from the sea water. Another siphon empties waste products back into the sea. As the animal grows in size, it chisels at the walls of its rocky home to expand them.

Despite their tough-guy attitude, piddocks are elusive little things, and their shells are (amazingly) so brittle that, once exposed to the ocean, they are quickly broken up – which means that you are unlikely to find a whole piddock shell unless it’s still inside the stone.

However, if you’re walking along a beach and you pick up a pebble that is riddled with small holes – or one that looks like a half-inch drill bit has gone right through it – you’re probably holding a fine example of their handiwork.